Electropolishing method



Feb. 17, 1959 E. R. BOWERMAN ETAL ELECTROPOLISHING METHOD Filed 001:. 26, 1954 Sheets-Sheet 1 IAV\ 1" I A I. YA AA/L'LY AL 5253 40 Y AVA L. A

A YBA IAM M YAIAYA VIIAVAA 8O YVAVAVAVWQVVYAVAVAVMEK 2o AYAVAVLVAYAYAYAAAAVAYAVA A16! I 9o mmigl gmwmvmmm YVAYLYIAYAYAYAYAYAHQAYAVALYMNA M W Y: A! o o lo" 2% 40 I00 4Y0 @0 so 70' so 50 4o 60 20 log o M A G %HYDROCHLORIC ACETIC,

' ANHYDRIDE ACID ' INVENTO wwmo a. am an" epwm a. sow MAN ATToRNEY Feb. 17, 1959 E. R. BOWERMAN ET AL ELECTROPOLISHING METHOD Filed 001;. 26, 1954 5 Sheets-Sheet 3 FIG. 5.

l2 %ACETIC ACID- WATER %HYDROCHLOR IC 7 ACETIC ACID ANHYDRIDE IN V EN TORS EDWARD B. SAUBEfi'TRE.

EDW\N R. EOWERMAN ATTQRNL) Feb. 17, 1959 E. R. BOWERMAN ETAL 2,874,104

ELECTROPOLISHING METHOD Filed Oct. 26, 1954 5 Sheets-Sheet 4 FIG. 4;

- vvvv ACETIC v WATER NV M vQG/io W W WWV so NWVMVMVVV VA 80 WWWWV 5. 20

\oo 90 ao 70/ 605 50 10 502 2o l0 0 HYDROCHLORIC ACETIC ACID ANHYDRIDE INVENTORS EDWARD a. SAUBESTRE. znwm a. BOWERMAN lay/ 701mm ATTORNEY Feb. 17, 1959 E. R. BOWERMAN ET AL 2,874,104

ELECTROPOLISHING METHOD Filed Oct. 26, 1954 5 Sheets-Sheet 5 FIG. 5.

% ACETIIC WATER ACID % HYDROCHLORIC ACETIC v ANHYDRIDE ACID IN VEN TORS v EDWARD B. SAUBESI'ILL EDWIN R. BOWERMAN ATTOtNI-Y chemically working metal parts obviating temperature of operation,

United States Patent ELECTROPOLISHING METHOD Application October 26, 1954, Serial No. 464,758 Claims. (Cl- 204-1405) The present invention relates to the electrolytic treatment of metals, and in particular to improved methods and baths for electropolishing metals. To advantage, molybdenum and nickel, and allied materials having molybdenum and nickel as essential ingredients, may be anodically polished.

Polishing metals and their alloys with electrolytes which are composed of mixtures of perchloric acid with organic materials, such as acetic anhydride or acetic acid, have been described in the literature. Although excellent results can be anticipated with acetic-perchloric electropolishing baths, such baths are exceptionally dangerous and have given rise to violently explosive reactions. The industrial hazards presented by such baths are discussed at length in an article appearing in Metal Finishing, November 1949 by Dr. Pierre AJacquet entitled The safe use of perchloric-acetic electropolishing baths. As set forth in the article, these perchloric-acetic baths may be handled with a fair degree of safety if rigid'control is maintained at all times over operating conditions. For example, the bath temperature must not be allowed to rise abovethe maximum prescribed operating-temperature; electrical shorts or sparks which may cause local overheating must be prevented; plastic and other organic materials must be excluded from the bath; and evaporation must not be allowedto change the bath composition into a formulation in the detonation region.

Although these controls are known to and practiced by those skilled in the art, the frequency of industrial accidents and explosions attests to the difficulties of maintaining such rigid control.

Apart from the ever-present risk of explosion, such perchloric-acetic electropolishing baths exhibit other short-comings which limit their industrial application and make difficult their successful use. Specifically, with respect topolishing action, the baths exhibit a narrow working range of current densities and temperatures, are made up of moderately expensive materials, and may have poor electrical conductivity.

Accordingly, it is an object of the present invention to provide improved methods and solutions for electroone or more or the aforesaid difficulties. Specifically, it is within the contemplation of thepresent invention to provide improved methods and electrolytes which are safe in use for electropolishing metallic components.

It is a still further object ofthe present invention to provide improved electrolytic solutions for polishing metal parts which are not extremely critical with respect to which facilitate electropolishing at room temperature, and which require little or no control over. ambient temperature. 7 It is a still further object of the present invention to provide electropolishing baths which leave treated metal surfaces bright, highly reflective and substantially free of films, such as notto require subsequent treatment.

.It is a still further object of the present invention to provide improved electropolishing baths capable of operating at comparatively high current densities .for achieving rapid leveling of surfaces being polished and having relatively broad operating ranges of anode current densities.-,

chloric 2,874,104 Patented Feb. 17, 1959 It is a still further object of the present invention to provide electropolishing baths having relatively broad ranges for the essential ingredients thereof, thereby assuring operativeness of the bath even though the composition may change somewhat during usage.

We have found that an electropolishing bath consisting essentially of an aqueous solution of hydrochloric acid and an additional ingredient selected from the group consisting of acetic anhydride and acetic acid is highly suitable for anodic polishing of metals, particularly molybdenum and nickel. The use of acetic anhydride-hydroacid solutions and acetic-hydrochloric acid solutions, each containing substantial amounts of hydrochloric acid, is contrary to the teachings in the art which indicate that the best polishing action is obtained in the absence of large amounts of hydrochloric acid.

To marked advantage, electropolishing solutions in accordance with the present invention function much like the known perchloric-acetic electropolishing baths, yet do not present the risk of explosive reaction. Additionally, the present baths are not sensitive to water content and operate successfully with large water concentrations, even of the order of 90% by volume. This contrasts to the conventional acetic-perchloric type baths which are extremely sensitive to water content and require fairly narrow permissible water concentration limits for polishing. Thus, although the makeup of the present baths may change during use, for example due to drag in and drag out, precautions need not be taken to maintain the water content of the baths within narrow concentration limits.

More generalized advantages are realized by employing electropolishing solutions according to the present inven tion, which include ease of formulation, good electrical conductivity which makes possible operation at relatively low voltages, use of commercial grade chemicals which facilitates formulation from relatively inexpensive materials, ability to operate at room temperature, relative insensitivity to changes in ambient temperature, high operating current densities which achieve rapid levelling and a wide operating range of current densities which makes possible electropolishing of irregular and recessed objects. Further, the presence of the chloride ion facilitates plating "ice out of the metal ion at the cathode, thus prolonging usetaken with the ful life.

A suggested mechanism for the operation of acetichydrochloricr electrolytes according to the'present invention is that the chloride ions of the electrolytes are anodically converted to another form, and that the modified chloride ions are effective in achieving electropolishing. Specifically, upon passage of current through the anode in a predetermined amount, the chloride ions are oxidized at the anode to form perchlorate ions in situ and as needed for electropolishing. This localized and in situ preparation of the perchlorate ions is effective in accomplishing the desired electropolishing action, ye does not present any hazard of explosion.

The above description, as well as further objects, advantages and features of the present invention will be best appreciated by reference to the detailed description of presently preferred baths and treatment methods, when accompanying drawings, wherein:

Figure 1 shows a ternary or triangular coordinate diagram illustrating nickel-electropolishing regions for the numerous electrolytes coming within the scope of the present invention;

Figure 2 is a ternary diagram illustrating the lower operating limit of current densities for nickel-electropolishing solutions within the regions prescribed in Figure 1;

Figure 3 isa ternary denum-electropolishing regions diagram illustrating the molybfor the numerous 91cc trolytes coming within the scope of the present invention;

tive proportions of electrolytes containing hydrochloric acid, acetic acid and water for electropolishing nickel, and allied materials having nickel as an essential constituent. The ternary diagram further illustrates electropolishing solutions which consist essentially of hydrochloric acid and acetic anhydride for electropolishing nickel and its alloys. The three coordinates of the ternary diagram represent water, which is present in increasing quantities along lines parallel to and progressively removed from the zero percent water axis 1i hydrochloric acid which is present in increasing quantities along lines parallel to and progressive! removed from the zero percent hydrochloric acid axis 12; and acetic acid which is present in increasing quantities along lines parallel to and progressively removed from the zero percent acetic acid axis 14. For the sake of convenience in illustration, a secondary zero percent water axis 1%) is illustrated offset from the main axis 10, along which secondary axis increasing quantities of hydrochloric acid are read from left to right using the numbers on the axis 10, and increasing quantities of acetic anhydride are read from right to left using the numbers on the axis 10'. The reading of such ternary diagrams is well known in the art and accordingly further description of such diagrams will be dispensed with.

The acetic-hydrochloric acid solutions suitable for electropolishing nickel are found in the regions approximately enclosed by the solid lines in the triaxial diagram of Figure 1, specifically by the solid curved line AB, the solid curved line BC, the solid curved line CD, the solid curved line DE, the solid straight line EF extending parallel to the zero percent acetic acid axis 14, the curved line PG, and the solid straight line GA coinciding substantially with the zero percent water axis 10.

Preferred compositions of nickel-electropolishing solutions with which best polishing action is obtained lie within the three lesser regions bounded by the dash-dash curved lines; a first preferred electropolishing region is bounded by the curved solid line AB, the segment BH of the curved solid line BC HA; a second preferred electropolishing region is bounded by the segment D1 of the solid line DE and the curved dash-dash line DI; and a third preferred electropolishing region .is bounded by the segment JG of the solid line FG, the segment GK of the zero percent water axis, and the curved dash-dash line KL As seen from an inspection of Figure 1, along the offset zero percent water axis 10 and between the limits A, G, acetic anhydride-hydrochloric acid electropolishing solutions have a composition varying from 42% acetic anhydride and 58% hydrochloric acid to 4% aceticanhydride and 96% hydrochloric acid.

In the regions of the ternary diagram of Figure 1, below and to the left of the curved lines AB and BC, no polishing action occurs and the surfaces under treatment remain dull; in the regions above and to'the right of the curved lines CD and DE there is dullness and pitting of the surfaces and in the regions to the right of the curved line FG pitting of the surfaces is observed.

The approximate coordinates of the points defining the overall and preferred nickel-electropolishing regions in Figure 1 are as follows:

A-0% water, 42% acetic acid and 58% hydrochloric acid.

and the curved dash-dash line close tothe region having lines into two operating ranges of B-l0% water, acetic acid, and 20% acid.

hydrochloric acid.

I48V2% water, 372% acetic acid, and

chloric acid.

J20% water, 2% acetic acid, and 78% acid.

K-0% water, 25% acetic acid, and

acid.

48 hydrohydrochloric hydrochloric Selection of a particular nickel-electropolishing solution coming within the overall or preferred regions depends upon practical considerations, such as the required width of the operating range and the power consumption. The wider the operating range, the more uniform will be the polishing action on recessed or irregularly shaped objects, while the lower the total amount of current required, the smaller the required power source. It is to be understood that no sharp transition occurs in the action of the bath as the outer limits of the defined electropolishing regions are approached. Rather, the limits define a threshold region wherein the polishing action may be other than optimum and/or where the power consumption becomes prohibitive.

Referring still further to Figure 1, there is shown a gradient line 16 legended 3000, which divides the overall nickel-electropolishing region enclosed in the solid anode current density. Specifically above and to the left of the gradient line 16, the permissible operating range is' greater than 3000 amperes per square foot, while below and to the right of the gradient line 16, the permissible operating range is less than 3060 amperes per square foot.

Referring now specifically to Figure 2, there is shown a ternary diagram illustrating the lower operating limit of current densities (stated in amperes per square foot) for nickel-electropolishing solutions according to the present in ention. By reference to Figure 2, the lower operating limit of current density can be ascertained for a given solution; and by reference to Figure 1, it is possible to as: certain the permissible operating range for the given solu- It is to be understood that the current density values for the lower operating limit in the respective regions are approximations and that said values vary as adjacent regions are approached. I

From the foregoing, the conjoint reading of the triaxial diagrams of Figures 1 and 2 to ascertain the operating range and the lower operating limit of current density for a selected nickel-electropolishing solution should be understood. The following example is set forth for the purposes of illustration: v

The point on the triaxial diagram of Figure l represented by the legend P indicates an electropolishing solution consisting essentially of 25 water, 40% acetic acid,

and 35% hydrochloric acid. For the solution P, which lies within the preferred elcctropolishing region ABHA the permissible operating range of current densities is in excess of 3000 amperes per square foot. Upon reference to the corresponding point P of Figure 2, it will be seen that the lower operating limit is between 750 and 1000 amperes per square foot. in that the point P lies rather a permissible lower operating range of 1000 to 1500 amperes per square foot, it will be appreciated that the lower operating limit should be se- 'ing limit of current lected toward the upper portion of the range. Thus a range of operation might be selected between a lower value of approximately 900 amperes per square foot and an upper value of at least 3900 amperes per square foot. By a similar procedure, reading along the offset zero percent water axis 10' in Figures 1 and 2 and lower operating limits and operating ranges may be determined for nickel-electropolishing solutions consisting essentially of acetic anhydride and hydrochloric acid.

Referring specifically to Figure 3 there is shown in the ternary diagram for acetic-hydrochloric acid solutions suitable for electropolishing molybdenum in accordance with the present invention. The ternary diagram of Figure 3 further illustrates electropolishing solutions which consist essentially of acetic anhydride and hydrochloric acid for electropolishing molybdenum and its alloys in accordance with the present invention.

The acetic hydrochloric acid solutions suitable for electropolishing molybdenum are found in the regions approximately enclosed by the solid lines in the triaxial diagram of Figure 3, specifically by the solid straight line LM extending substantially'parallel to the zero percent water axis, the curved line MN, the curved line NO, the curved line OR and the curvedsolid line RL.

The acetic anhydride-hydrochloric acid solutions suitable for electropolishing molybdenum are found along the ofiset zero percent water axis 10 along the solid line ST.

The approximate coordinates of the points defining the molybdenurn-electropolishing regions in Figure 3 are as follows:

L9 water, 33% acetic acid, and 58% hydrochloric acr acid.

N-20% water, 72% acetic acid, and 8% hydrochloric acid.

acid.

R-93% water, 3.5% acetic acid, and 3.5% hydrochloric acid.

S% water, 56%

chloric acid.

T-0% water, 28% acetic anhydride,

chloric acid.

As shown in Figure 3, the molybdenum-electropolishing region is divided by gradient lines, generally designated by the numeral 18, each of which is appropriately legended. The gradient lines 18 divide the overall molybdenum-electropolishing region into a number of lesser regions having difierent operating ranges of anode current densities. Again it is to be appreciated that no sharp transition occurs in the values and ranges of anode current densities for the several regions of Figure 3; rather the ranges vary as the respective gradients are approached and accordingly the selection of an operating range should be made in.view of the relative location of the solution in respect to adjacent regions of difi'erent operatingranges. a

Reference will now be made to Figure 4, wherein there is shown a ternary diagram illustrating the lower operating limit of anode current densities for molybdenum-electropolishing solutions according to the present invention. By reference to Figure4'of the drawing, the lower operatdensity can be ascertained for a given solution; and by reference to Figure 3, it is possibleto ascertain the permissible operating range for the given solution. It is to-be-understood that the current density values for the lower operating limit in the respective regions enclosed by the various gradients are approximations and the values may vary as the limits of any given region is approached.

In that the conjointreading of the triaxial diagrams of Figures 3 and 4 to ascertain the operating" range and water, 33% acetic acid, and 12% hydrochloric acetic anhydride, and 44% hydroand 72% hydro- Acetic Hydrochloric Solution I Acid Acid (Specific Water (99.5%) gravity 1.18)

The following examples are illustrative formulations consisting essentially of acetic anhydride and hydrochloric acid for electropolishing nickel and molybdenum in accordance with the present invention, which are listed in percentages by volume:

I Acetic Hydrochloric Solution Anhy- Acid (Specific Water I dride gravity 1.18)

Referring now specifically to Figure 5, there is shown a ternary diagram which illustrates the common electropolishing regions for both nickel and molybdenum, with acetic acid-hydrochloric acid solutions and with acetic anhydride-hydrochloric acid. solutions in accordance with the present invention. The nickel-electropolishing regions are outlined by the line generally designated by the reference numeral 20,. coinciding with the heavy line showing of Figures 1 and 2; the molybdenum-electropolishing region is outlined by the line generally designated by the line 22 and coinciding with the heavy line showing of Figures 3 and 4; and the common region is enclosed withinthe solid heavy line 24 and vertically hatched. All solutions lying within the area enclosed by the heavy line 24 arefsuitable for electropolishing either nickel or molybdenum. As can be readily appreciated by comparing the, regions enclosed respectively by the lines 20, 22, the molybdenum-electropolishing region lies substantially within the nickel-electropolishing region and accordingly molybdenum-electropolishing solutions in most instances are suitable for electropolishing nickel. f f The acetic anhydride-hydrochloric acid solutions suitable for electropolishing both nickel and molybdenum *lie alongthe offset zero percent water axis 10 between 60 the points A, T. Any solution lying along solid line A'T coinciding with the oitset zero percent water axis is suitable for electropolishing both nickel and molybdenum with acetic 'anhydride-hydrochloric acid.

Specifically, the common region of Figure 5 is enclosed by the solid black line UV, the solid curved line' VW, thesolid curved line WX, and solid curved line XY, the solid curved line YZ, and the substantially straight solid line ZU. I y

The approximate coordinates of the points defining the common electropolishing region of Figure 5 are as follows: a i

U- l2% water/% acetic acid, and 18% hydrochloric W'93% water, 3.5% acetic acid, and 3.5%

ric acid.

X72% water, 10% acetic acid.

Y50% water, acetic acid, and 45% acid.

Z--% water, 30% acetic acid, and 60% acid.

As can be seen by reading along the offset zero percent water axis, the common region for acetic anhydridehydrochloric acid solution solutions lie between the point hydrochloacid, and 18% hydrochloric hydrochloric hydrochloric A, previously designated as having the coordinates 42% acetic anhydride and 58% hydrochloric acid, and the point T previously designated as having the coordinates 28% acetic anhydride and 72% hydrochloric acid.

For the illustrative solution P", previously pointed out as consisting essentially of 25% water, 40% acetic acid and 35% hydrochloric acid, both nickel and molybdenum may be polished with equal success. Depending upon the material to be treated, the operating range of current densities and the lower limit of current density is ascertained by conjoint reading of the appropriate figures in the drawing.

The accompanying ternary diagrams serve to illustrate the relative proportions of hydrochloric acid and either acetic anhydride or acetic acids which may be suitable for anodic polishing of nickel, molybdenum and their respective alloys. However, the solutions may include other ingredients such as other acids, metallic salts and contamination due to normal operation. For example, in making up an electrolyte suitable for the anodic polishing of nickel, the bath corresponding to point P might be selected in that it lies within one of the preferred areas of the nickel electropolishing region. During the continuous use of such bath in the electropolishing of nickel, the composition may change. This changing composition may be due to the anodic dissolution of nickel into the bath, and the possibility of loss or addition of water content due to evaporation, absorption, drag-in, and drag-out. Despite such changes in the composition as may occur during continued use, if the relative percentages of hydrochloric acid, acetic acid and water, expressed in percentages by volume, remain in the preferred or less preferred region of the diagram of Figure l, the bath will continue to operate satisfactorily. This is to extreme advantage, especially when compared to acetic-perchloric type solutions which are exceptionally sensitive to water content and the presence of foreign organic substances. For the acetic-perchloric type solutions if the permissible concentration limits are exceeded, or if contamination is introduced into the solution, there is the risk of a violent explosive reaction. For example, by reference to the ternary diagram in'the mentioned article, it will be seen that evaporation can convert a comparatively safe acetic-perchloric formulation into a composition in the detonation region.

Consideration of the probable mechanisms involved herein indicates that the chloride ions ofthe hydrochloric acid are anodically converted to another form, this modified form of the hydrochloric acid is effective in the electropolishing action. Thus it would appear that the chloride ion, which is readily oxidiz'able, combines with nascent or free oxygenliberated at the cathode such that once the reaction is started perchlorate ions are formed at the anode as needed for electropolishing. Thus the metal of the anode goes into solution causing electropolishing. Accordingly, although the foregoing description is based upon compositions for electropolishing nickel and molybdenum and their respective alloys, equally within the contemplation of this disclosure is the polishing of other metals heretofore polished by acetic-perchloric type baths which relied upon the perchlorate ion for polishing action. v

Numerous modifications and substitutionsin the presscut process and bath will occur to those skilled in the art and accordingly the appended claims should be given a latitude and interpretation consistent with the present disclosure; at times certain features of the invention will be used without corresponding use of other features.

We claim:

1. The method of electropolishing a nickel part incl1lding the steps of making said part the anode in an electrolytic solution consisting essentially of hydrochloric acid and acetic acid which are present in relative percentages by volume lying within the areas defined approximately in the ternary diagram of Figure l by the solid lines AB,

C, CD, DE, EF, PG and GA, and passing electric current through said anode in an amount sufficient to obtain electropolishing action of said surfaces.

2. The method of electropolishing a molybdenum part including the steps of making said part the anode in an aqueous electrolytic solution consisting essentially of hydrochloric acid and acetic acid which are present in relative percentages by volume lying within the area defined approximately in the ternary diagram of Figure 3 by the solid lines LM, MN, NO, OR, and RL, and passing electric current through said anode in an amount sufiicient to obtain electropolishing action of said surfaces.

3. The method of anodically polishing articles having surfaces of a metal selected from the group consisting of nickel and molybdenum, including the steps of making one of said articles the anode in an electrolytic solution containing as essential ingredients hydrochloric acid and acetic acid, said percentages being by volume and the relative percentages of said ingredients lying within the area defined in Figure 5 of the accompanying drawings by the lines UV, VW, WX, XY, YZ and ZU, and passing electric current through said anode in an amount sufficient to obtain electropolishing action of said surfaces.

4. The method of anodically polishing an article having nickel surfaces including the steps of making said article the anode in an electrolytic solution containing as essential ingredients from 2.5 to 96 percent hydrochloric acid and the further ingredient selected from the group consisting of acetic anhydride and acetic acid, said percentages being by volume and the relative percentages of said ingredients lying within the region defined in Figure l of the accompanying drawings by the lines AB, BC, CD, DE, EF, PG and GA, and passing an electrolytic current through said anode in an amount suificientto obtain an electropolishing action of said n'ckel surfaces as determined by Figure 2 of the accompanying drawings.

5. The method of anodically electropolishing an article having molybdenum surfaces including the steps of making said article the anode in an electrolytic solution containing as essential ingredients from 2.5 to 63 percent hydrochloric acid and a further ingredient selected from the group consisting of acetic acid and acetic anhydride, said percentages being by volume and the relative percentages of said ingredients lying within the region defined in Figure 3 of the accompanying drawings by the lines LM, MN, NO, OR and RL, and passing an electric current through said anode in the amount suflicient to obtain electropolishing action of said molybdenum surfaces as determined by Figure 4 of the accompanying drawings.

References Cited in the file of'this patent UNITED STATES PATENTS I p OTHER REFERENCES Academic des scienceglcomptes Rendus, vol-233, July 1951, pages -162, article by Tajima et al. 

1. THE METHOD OF ELECTROPOLISHING A NICKEL PART INCLUDING THE STEPS OF MAKING SAID PART THE ANODE IN AN ELECTROLYTIC SOLUTION CONSISTING ESSENTIALLY OF HYDROCHLORIC ACID AND ACETIC ACID WHICH ARE PRESENT IN REATIVE PERCENTAGES BY VOLUME LYING WITHIN THE AREAS DEFINED APPROXIMATELY IN 